nutrient cycling (nutrient cycle)

Nutrient cycling is the continuous biogeochemical movement of essential plant nutrients — primarily nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and various micronutrients — through soil, plants, decomposer organisms, and back to soil. The cycle has multiple pathways: uptake by plant roots, incorporation into plant biomass, return to soil as crop residue or manure, microbial decomposition, mineralisation into plant-available ion forms, and re-uptake by the next crop cycle.

In intensive Indian agriculture, the natural nutrient cycle is heavily disrupted. Roughly 95% of harvested grain and biomass leaves the field — fed to humans and livestock in cities and exported to markets — while only a small fraction of the nutrients consumed eventually returns to the original field. This one-way flux is replaced by synthetic fertilisers (urea, DAP, MOP), which restore N, P, and K but not the organic carbon, micronutrients, or soil-biology drivers that natural cycling provides. The result is the well-documented decline in soil organic carbon across the Indo-Gangetic plain, micronutrient deficiencies in zinc, iron, and boron, and falling responsiveness to fertiliser application.

Biogas plants and composting operations offer a route to partially restore the cycle. Anaerobic digestion captures organic matter and nutrients from agricultural residues, animal manure, and municipal organic waste, and the resulting digestate — applied as Liquid Fermented Organic Manure or Phosphate-Rich Organic Manure under FCO 1985 norms — returns N, P, K, organic carbon, and micronutrients to farmland. Closed-loop arrangements, where a CBG plant collects residue and manure from surrounding farms and returns digestate to the same farms, are increasingly promoted under the SATAT scheme and the Galvanising Organic Bio-Agro Resources Dhan (GOBAR-Dhan) programme. The trade-off versus synthetic fertiliser is that digestate has lower nutrient concentration by volume — typically 1.5–3.5% N on dry matter versus 46% N in urea — but adds soil organic matter and biological activity that synthetic fertilisers cannot. Achieving 30–40% substitution of synthetic N with digestate-derived N is the realistic medium-term target identified in NITI Aayog's biogas roadmap.